The present invention relates to a method for producing a reverse type silicon thin film transistor used in an active matrix type liquid crystal display device or a photosensor.
Research and development of a reverse staggered type silicon thin film transistor has been conducted in order to use the same in an active matrix type liquid crystal display device and a photosensor.
Such a reverse staggered type silicon thin film transistor is shown in FIG. 3 and an explanation will now be given as to the method of producing the same.
In a conventional production method, a gate insulating layer 3 is formed on a substrate 1 having a gate electrode 2. An intrinsic silicon layer 4 containing almost no impurities in the transistor-forming part thereof is then formed on gate insulating layer 3, and an n-type silicon layer 5 containing n-type impurities is formed on intrinsic silicon layer 4. On the thus-formed incomplete transistor, n-type silicon layer 5 is etched by using source electrode 6 and drain electrode 7 as the mask, so as to render source electrode 6 and drain electrode 7 non-conductive by means of n-type silicon layer 5.
Since fluctuation of the degree of etching occurs in substrate 1 when etching n-type silicon layer 5, intrinsic silicon layer 4 is simultaneously etched as shown in FIG. 3 to remove the n-type silicon layer thereat.
When forming n-type silicon layer 5 on intrinsic silicon layer 4, since the n-type impurities in n-type silicon layer 5 diffuse into intrinsic silicon layer 4, the upper part of intrinsic silicon layer 4 is formed as an n-type layer.
Also, there is almost no selectivity of etching between n-type silicon layer 5 and intrinsic silicon layer 4. Therefore, in order to remove the n-type upper part of silicon layer 4 in all transistors on the substrate, it is necessary to etch silicon layer 4 down to a considerabe depth.
However, when the etching is excessive, due to damage or the like to silicon layer 4, the on-current is lowered, and/or source electrode 6 and drain electrode 7 overhang silicon layer 4 at the terminal parts of source electrode 6 and drain electrode 7 such that cracks may form therein.
On the other hand, when the etching is not carried out sufficiently, complete removal of the n-type upper part of silicon layer 4 is impossible, so that the off-current increases.
In the conventional production method, control of the above-described etching is very difficult, such that a transistor with good reproducibility cannot be formed.